Scientists surveying the Great Barrier Reef say that it has suffered the worst coral die-off ever recorded after being bathed this year in warm waters that bleached and then weakened the coral. About two-thirds of the shallow-water coral on the reef's previously pristine, 430-mile northern stretch was dead. Only a cyclone that lowered water temperatures by up to three degrees Celsius in the south saved the lower reaches of the 1,400-mile reef from damage. [Source: N.Y. Times]